The present invention relates to the field of electrophotographic reproduction and, more particularly, to dual layer organic photoconductive elements such as belts, drums, webs, or the like, which are used in machines such as copying machines. U.S. Pat. Nos. 3,615,415, 3,824,099 and 4,150,987 have a discussion of the general field of the present invention and disclosures of some of the specific materials and procedures over which the present invention represents an improvement.
Aforementioned U.S. Pat. Nos. 3,824,099 and 4,150,987 relate to dual layer organic photoconductive elements comprising a conductive substrate such as a conductive paper, metallized plastic film or metal plate, supporting two basic layers, namely a charge-generating layer and a charge-transport layer. Either layer may be adjacent the conductive substrate, and an adhesive bonding layer may be interposed to bond the charge-generating layer.
During use, the photoconductive element is charged, exposed to light passed from the light-reflective areas of an imaged original sheet to conduct away the charge in the exposed areas, and the charge-retaining or unexposed areas are either "inked" with electroscopic toner which is transferred to a copy sheet or are first transferred to a copy sheet and then "linked" with toner thereon. The toner is fused on the copy sheet to form fixed images corresponding to the images present on the original sheet.
Dual layer organic photoconductive elements were developed in order to provide extremely fast response to light exposure within the visible range of the spectrum. Preferred materials in this respect are diane blue dyes as the charge-generating organic chemicals of the charge-generating layer and p-type hydrazones or triaryl pyrazolines as the organic chemicals in the charge-transport layer.
Diane blue dyes have the general moiety structure as follows: ##STR1## wherein Z is a substitutent group on the phenyl ring. Preferably, Z is selected from a group consisting essentially of hydrogen, alkyl, alkoxy and chlorine groups, more preferably hydrogen, methyl, methoxy and chlorine groups; and most preferably a chlorine group. Z is preferably attached to the ring in the 2 or 6 position. The most preferred diane blue, Chlorodiane Blue, has chlorine as the Z group in the 2 position.
Chlorodiane Blue is a preferred charge-generating material because it has good stability and provides photoconductive elements having a desired shelf life or duration of storage prior to breakdown and deterioration or loss of its charge-generating properties. Chlorodiane Blue also has superior light sensitivity and charge-generating properties over a broad portion of the visible light range, i.e., between about 400 m.mu. and 700 m.mu.. However, Chlorodiane Blue lacks adequate light sensitivity in the near-infrared range, i.e., between about 780 m.mu. and 1150 m.mu..
Squarylium dyes are described in U.S. Pat. No. 3,824,099 as providing charge-generating layers which are panchromatic out to about 7000 Angstrom units and which are responsive to light having wave lengths up to 9000 Angstrom units. However, we have found that photoconductive elements prepared according to U.S. Pat. No. 3,824,099, and containing squarylium dye as disclosed in said Patent as the charge-generating material, have an insignificant amount of negative charge acceptance, and therefore are substantially useless as photoconductive elements for duplication purposes. However, we discovered that by combining a certain amount of the squarylium dye with a diane blue dye, such as Chlorodiane Blue, charge-generating layers can be produced which have the desired negative charge acceptance and which are also sensitive to near-infrared radiation.